The sorting or marking of livestock by weight to determine which animals of a group and variety are sufficiently developed to be marketed is a common practice. Those animals having a weight above a pre-calculated amount based upon the type of animal and present market conditions in representing the best financial return thereon are selected by the producer for sale. Certain types of livestock, such as pigs, hogs, and the like, have an unpleasant disposition and oftentimes exhibit hostile tendencies toward their caretaker. The sorting of these types of animals can be very dangerous and is quite a time consuming task. Further, the sorting process often excites the animals which can result in weight loss and/or physical injury to the animals. Devices for sorting livestock, such as those disclosed in U.S. Pat. Nos. 262,373, 2,715,387, and 4,006,717 are generally not automated and/or require an outside source of power to operate the device. Clearly, the use of external power supplies to operate a sorter for livestock is neither convenient, economical, nor safe for use in most applications. Further, such devices tend to arouse the animals, thereby causing weight loss and other physical injuries which reduce the economic value of the animal.